Variable capacitor single-electron device

ABSTRACT

The present invention provides a single-electron transistor device  100 . The device comprises a source  105  and drain  110  located over a substrate  115  and a quantum island  120  situated between the source and drain, to form tunnel junctions  125, 130  between the source and drain. The device further includes a fixed-gate electrode  135  located adjacent the quantum island  120 . The fixed-gate electrode has a capacitance associated therewith that varies as a function of an applied voltage to the fixed-gate electrode. The present invention also includes a method of fabricating a single-electron device  300 , and a transistor circuit  800  that include a single-electron device  810.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/448,673, entitled, “A SUSPENDED GATE SINGLE ELECTRONDEVICE,” to Christoph Wasshuber, filed on May 30, 2003, now U.S. Pat.No. 6,844,566 which is commonly assigned with the present invention, andincorporated by reference as if reproduced herein in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed in general to the manufacture ofsemiconductor devices, and, more specifically, to a single electrondevice and method of fabrication thereof.

BACKGROUND OF THE INVENTION

The continuing demand for increasing computational power and memoryspace is driving the miniaturization of integrated circuits. To sustainprogress, miniaturization will soon be driven into the nanometer regime.Unfortunately, conventional devices cannot be scaled downstraightforwardly, because of problems caused by parasitic resistances,scattering and tunneling.

Single-electronics offers solutions to some of the problems arising fromminiaturization. Single-electronic devices can be made from readilyavailable materials and can use as little as one electron to define alogic state. Unlike conventional devices, single-electron devices showimproved characteristics when their feature size is reduced. Thisfollows from the fact that single-electron devices are based on quantummechanical effects that are more pronounced at smaller dimensions.Single-electron devices also have low power consumption and thereforethere are less energy restrictions to exploit the high integrationdensities that are possible with such devices.

The practical implementation of single-electronic devices capable ofreproducibly defining a logic state remains problematic, however. Forinstance, it is desirable to develop process technology conducive to themass production of nanometer scale single-electron devices structuresand for such devices to operate at room temperature. Much more importantthan mass production and room temperature operation, however, is thesensitivity of single-electron devices towards random background chargeeffects.

A random background charge can alter the Coulomb blockade energy,thereby altering the operating characteristics of the device. Forinstance, a trapped or moving charge in proximity to a single-electrontransistor (SET) logic gate could flip the device's logic state, therebymaking the output from the device unreliable at any temperature. Inaddition, background charge movement can cause the device'scharacteristics to shift over time.

Previous attempts to reduce the random background charge dependence ofsingle-electronic devices have not been entirely successful. Efforts tofind impurity-free fabrication techniques have not lead to devices thatare sufficiently free of random background charge. Adding redundancyinto the logic circuit is considered to be ineffective, especially inthe presence of high background charge noise levels. Anoperating-point-refresh to adjust the bias conditions of the device isalso not considered to be an efficient solution. Accordingly,single-electronic logic devices have heretofore been considered to beimpractical due to their sensitivity to random background chargeeffects, and the consequent instability of the device's logic state.

Accordingly, what is needed in the art is a single-electron device andmethod of manufacturing thereof that overcomes the above-mentionedproblems, and in particular minimizes random background charge effectson device function.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides a single-electron transistor device. Thedevice comprises a source and drain located over a substrate and aquantum island situated between the source and drain, to form tunneljunctions between the source and the drain. The device further includesa fixed-gate electrode located adjacent the quantum island, thefixed-gate electrode having a capacitance associated therewith thatvaries as a function of an applied voltage to the fixed-gate electrode.

In another embodiment, the present invention provides a method offabricating a single-electron device. The method includes forming asource and drain located over a substrate. The method also comprisesplacing a quantum island between the source and drain, wherein thequantum island forms tunnel junctions between the source and the drain.The method also includes forming the above-described fixed-gateelectrode adjacent the quantum island.

Yet another embodiment of the present invention is a transistor circuit,comprising a single-electron device comprising a source, drain, quantumisland and fixed-gate electrode as described above, and a metal-oxidesemiconductor field-effect transistor (MOSFET) coupled to thesingle-electron device. The MOSFET is configured to amplify a draincurrent from the single-electron device.

The foregoing has outlined preferred and alternative features of thepresent invention so that those of ordinary skill in the art may betterunderstand the detailed description of the invention that follows.Additional features of the invention will be described hereinafter thatform the subject of the claims of the invention. Those skilled in theart should appreciate that they can readily use the disclosed conceptionand specific embodiment as a basis for designing or modifying otherstructures for carrying out the same purposes of the present invention.Those skilled in the art should also realize that such equivalentconstructions do not depart from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read with the accompanying FIGUREs. It is emphasized that inaccordance with the standard practice in the semiconductor industry,various features may not be drawn to scale. In fact, the dimensions ofthe various features may be arbitrarily increased or reduced for clarityof discussion. Reference is now made to the following descriptions takenin conjunction with the accompanying drawings, in which:

FIGS. 1A and 1B illustrate cross-sectional and top views of an exemplarysingle-electron transistor device of the present invention;

FIG. 2 illustrates a top view of an alternative single-electrontransistor device of the present invention;

FIGS. 3 to 7 illustrate cross-sectional and top views of selected stepsin an exemplary method for fabricating a single-electron deviceaccording to the principles of the present invention; and

FIG. 8 presents a circuit diagram of an exemplary transistor circuit ofthe present invention.

DETAILED DESCRIPTION

The present invention recognizes the advantages of using single-electrondevices that circumvent random background charge effects by usingCoulomb oscillations to store and transmit logic states. The termCoulomb oscillations, as used herein refers to the periodic change inthe drain current (Id) for increasing gate voltage (V_(G)) in asingle-electron device. Unlike the Coulomb blockade, the Coulomboscillation frequency is independent of random background charges.

The present invention further recognizes that the Coulomb oscillationfrequency in a single-electron device can be modulated by changing agate capacitance to the device. Moreover, a change in the logic state ofthe single-electron device can be accomplished by changing the gate'scapacitance used to change the Coulomb oscillation frequency. Thus,single-electron devices that can store and transmit logic states bychanging the Coulomb oscillation frequency are able to functionsubstantially independently of random background charge effects.

As disclosed in U.S. patent application Ser. No. 10/448,673, the gate'scapacitance can be changed using a movable electrode. By contrast, inthe present invention, a fixed-gate electrode is used. The termfixed-gate electrode as used herein refers to an electronic componentwith no moving parts and whose capacitance is configured to change as afunction of an applied voltage. That is, the fixed-gate electrode has acapacitance associated therewith that varies as a function of a voltageapplied to the fixed-gate electrode.

A fixed-gate electrode has several advantages over a moveable electrode.First, the ease of manufacturing the single-electron device issimplified because it is easier to build a fixed-gate electrode than amoveable electrode. Second, for a movable electrode, process flowvariables are more limited than for a fixed-gate electrode. As anexample, if one wishes to switch capacitance in the GHz range, then onehas to make the moveable electrode small and light enough to ensure thatmechanical frequencies are small enough to reach the GHz range. Suchprocess considerations are not an issue when using fixed-gateelectrodes. Third, movable electrodes can be less reliable thanfixed-gate electrodes, because components that are moving often are moreprone to fracture or breakage.

One embodiment of the present invention is shown in FIGS. 1A and 1B,which respectively illustrates cross-sectional and top views of anexemplary single-electron transistor device 100. Turning first to FIG.1A, the single-electron transistor device 100 comprises a source anddrain 105, 110 supported by a substrate 115. While it is shown that thesource and drain 105, 110 are located on the substrate 115, otherembodiments might provide the source and drain 105, 110 being locatedwithin the substrate 115. The substrate 115 can comprise anyconventional semiconductor material, such as silicon. A quantum island120 is located between the source and drain 105, 110 and forms tunneljunctions 125, 130 between the source and drain 105, 110.

As illustrated in FIG. 1B, the fixed-gate electrode 135 is locatedadjacent the quantum island 120. In a preferred embodiment, a dielectricmaterial 140 is located between the fixed-gate electrode 135 and thequantum island 120. A second dielectric material 142 can also be locatedbetween the quantum island 120 and the source and drain 105, 110. Thesecond dielectric material 142 can have the same or differentcomposition as the dielectric material 140.

The term, quantum island 120, as used herein, refers to the structurebetween the source and drain 105, 110 that facilitates the movement ofdiscrete electron tunneling from the from the source 105 to the island120 and from the island 120 to drain 110. Those skilled in the art arefamiliar with discrete electron tunneling and with other terms used torefer to the quantum island 120, such as a quantum dot, a grain, aparticle or a node.

With continuing reference to FIGS. 1A and 1B, for certain conditions andsizes of the quantum island 120, a voltage bias applied to thefixed-gate electrode 135 polarizes the tunnel junctions 125, 130. This,in turn, changes the Coulomb blockade energy, which is given bye²/2C_(Σ), where e is the electric charge on one electron, and C_(Σ) isthe total capacitance coupled to the quantum island 120. Preferably, thetemperature is low enough, and the quantum island 120 is small enough,that the Coulomb blockade energy is large compared to the ambientthermal energy kT (i.e., e²/2C_(Σ)>>kT). Under such conditions, changingthe Coulomb blockade energy facilitates tunneling of one or morediscrete electrons as described above.

As noted above, the Coulomb oscillation frequency of the drain currentcan be modulated by changing the total capacitance coupled to thequantum island 120. One component of the total capacitance is the gatecapacitance (C_(G)), the capacitance between the fixed-gate electrode135 and the quantum island 120. The periodicity of the Coulomboscillation is given by e/C_(G). In the present invention, thefixed-gate electrode 135 is configured to change a variable capacitance(C_(GV)) between the quantum island 120 and the fixed-gate electrode 135when a voltage (V_(G)) is applied to the fixed-gate electrode 135. Asfurther discussed below, there can also be a constant capacitance(C_(GC)) between the quantum island 120 and the fixed-gate electrode135. Changing C_(GV) results in a change in the Coulomb oscillationfrequency, which, in turn, can be use to encode logic states. That is,the gate capacitance (C_(G)), and hence logic states, can be changed byvarying the capacitance of the fixed-gate electrode 135. In certainembodiments for instance, it is desirable to apply one of two V_(G)values, corresponding to binary-encoded information, to the fixed-gateelectrode 135. The change in V_(G) preferably causes a large change inC_(G). Preferred Coulomb oscillation frequencies can range from about 1MHz to about 50 GHz.

There are numerous configurations available for the fixed-gate electrode135. In some preferred embodiments, for instance, as shown in FIGS. 1Aand 1B, the fixed-gate electrode 135 comprises a pn junction electrode.The pn junction electrode comprises silicon or polysilicon or otherconventional materials having an n-doped region 145 and a p-doped region147 with a depletion layer 150 there between. As well-known to thoseskilled in the art, a pn junction electrode comprises a capacitor. Forthe fixed-gate electrode 135 illustrated, the n-doped region 145 is onecapacitor plate, the p-doped regions 147 is a second capacitor plate,and the depletion layer 150 in between these plates 145, 147 can store acharge. The capacitance between the n-doped regions 145 and p-dopedregions 147 of the pn junction can be increased or decrease by applyinga voltage to the fixed-gate electrode 135 to thereby change the width155 of the depletion layer 150. For instance,in some cases, thecapacitance can be decreased by applying a backward bias to thefixed-gate electrode 135 to thereby decrease the width 155 of thedepletion layer 150. In other cases, the capacitance can be increased byapplying a forward bias to thereby increase the width 155 of thedepletion layer 150.

Another advantageous embodiment of the present invention is shown inFIG. 2, which illustrates a top view of an alternative single-electrontransistor device 200. FIG. 2 uses the same reference numbers to depictanalogous device components to that shown in FIGS. 1A and 1B. In theembodiment of the top view shown in FIG. 2, the fixed-gate electrode 135comprises a depletion electrode, such as a metal oxide semiconductorcapacitor (MOSCAP). In such embodiments, the fixed-gate electrode 135comprises a first body 205 adjacent the quantum island 120 and a secondbody 210 adjacent the first body 205. The first and second bodies 205,210 are separated by an insulator 215, the insulator comprising adielectric material such as silicon oxide.

In some advantageous configurations, the first body 205 comprises asemiconductor material, such as silicon, or more preferably dopedsilicon. The second body 210 can comprise a metal such as titanium,doped polysilicon, a semiconductor material such as doped silicon orcombinations thereof. The first body 205 further comprises a channelregion 220 located adjacent the insulator 215 and, in some embodiments,is between source and drain electrodes 225, 230 that are also located inthe body 205, thereby forming a MOSCAP. One of ordinary skill in the artwould understand how the charge state of the channel region 220 can bedepleted, accumulated or inverted as a function of the voltage V_(G)applied to the second body 210, either alone or in combination withvoltages applied to the source and drain electrodes 225, 230. Oneskilled in the art would also understand how switching the channelregion 220 between these charge states causes a corresponding change inthe capacitance of the fixed-gate electrode 135. Additionally, inembodiments where the second body 210 also comprises a semiconductormaterial, the fixed-gate electrode 135 can be operated in the oppositedirection as described above. That is, a voltage applied to the firstbody 205 can result in the charge state of a channel region of thesecond body 210 being depleted, accumulated or inverted, with acorresponding change in the capacitance of the fixed-gate electrode 135.

Other configurations of the fixed-gate electrode 135 are also within thescope of the present invention, so long as the capacitance of thefixed-gate electrode 135 can be varied by applying a voltage V_(G) tothe electrode. Moreover, the fixed-gate electrode 135 can comprise oneor more pn junction electrodes, one or more depletion electrodes or acombination of pn junction and depletion electrodes.

As mentioned above, in addition to the variable capacitive componentC_(GV), the fixed gate electrode 135 also has a constant capacitivecomponent C_(GC). Returning now to FIGS. 1A and 1B, the magnitude ofC_(GC) is a function of the distance 160 separating the quantum island120 and the fixed gate electrode 135. This distance 160 can beadvantageously adjusted to provide the desired change in C_(G) andcorresponding change in the Coulomb oscillation frequency of the device100. In certain embodiments, for example, the distance 160 between thefixed-gate electrode 135 and the quantum island 120 is between about 1nanometer and about 1000 nanometers, and more preferably between 10 and100 nanometers.

The shape of the fixed-gate electrode 135 can also be altered toincrease or decrease C_(GV) and C_(GC). For example, as illustrated inFIG. 1B, the fixed-gate electrode 135 can have a “T” shape, comprising afirst body 165 and second body 170. As shown in FIG. 1B, the first andsecond bodies 165, 170 can be united, by virtue of being patterned fromthe same material, for example. In the embodiment depicted in FIG. 1B,the first body 165 has long dimension 175 adjacent to the quantum island120, and the second body 170 has a long dimension 180 perpendicular tothe long dimension 175 of the first body 165. In some cases, the firstbody 165 includes the p-doped region 145, n-doped region 147 anddepletion zone 150. It would be apparent to one of ordinary skill in theart that changing the shape of other types of fixed-gated electrodes135, such as a depletion gate electrode, would similarly alter C_(G).

As noted above, the Coulomb oscillation frequency of the drain currentof the single electron device, such as the devices 100, 200 presented inFIGS. 1A–1B and 2, respectively, can be modulated by changing C_(Σ), ofwhich C_(G) is only one component. C_(Σ) is also a function of the fixedcapacitances between the source 105 and quantum island 120 (C_(S)) thedrain 110 and quantum island 120 (C_(D)). Because these capacitances arein series, C_(Σ) will be given by the sum of the reciprocals of C_(GV),C_(GC), C_(S) and C_(D), as well as any other capacitances associatedwith the quantum island 120. In certain embodiments, it is advantageousto adjust these capacitances so that 1/C_(GV) is approximately equal tothe sum of reciprocals of the constant capacitances (e.g.,1/C_(GV)˜(1/C_(GC)+1/C_(S)+1/C_(D))). In such embodiments, changingC_(GV) can cause a substantial change in C_(Σ) and hence in the Coulomboscillation frequency of the drain current of the single electron device100, 200.

Thus, with continuing reference to FIG. 1B, C_(Σ) can be altered bychanging the gap 185 between the source 105 and quantum island 120, thedrain 110 and quantum island 120, with resultant changes in C_(S) andC_(D), respectively. Choosing the distance of the gap 185 is tempered,however, by the requirement to form tunnel junctions 125, 130. The sizeof gap 185 needed between the source and drain 105, 110 and the quantumisland 120 to form tunnel junctions 125, 130 is well understood by thoseskilled in the art. For example, in some embodiments, the tunneljunctions 125, 130 are separated by a gap 185 of between about 1nanometer and about 1000 nanometers.

C_(Σ) also depends on the materials that the component parts of thesingle electron transistor 100 are made of. The source and drain, 105,110 quantum island 120 and fixed-gate electrode 135, can be made of thesame or different conventional materials. Such materials include, butare not limited to, silicon, GaAs heterostructures, metals,semiconductors, carbon nanotubes, or single molecules. In certainpreferred embodiments, the source and drain 105, 110 and the quantumisland 120 comprises doped polysilicon and the fixed-gate electrode 135comprises doped silicon.

C_(Σ) additionally depends on the choice of dielectric material 140 andthe second dielectric material 142. In certain preferred embodiments,the dielectric materials 140, 142, are both gases, such as air.Alternatively, the dielectric materials 140, 142 can be other gaseshaving a high dielectric constant, as well as a liquid or solid having ahigh dielectric constant (e.g., about the same or greater than thedielectric constant of air). In some preferred embodiments, one or bothof the dielectric materials 140, 142, comprise silicon dioxide, formedby oxidizing a constriction in a silicon wire that also serves as thesource and drain 105, 110 and quantum island 120. In other embodiments,one or both of the dielectric materials 140, 142, comprises aluminumoxide, which may be similarly formed by oxidizing a constriction in analuminum wire that also serves as the source, drain and quantum island.

The single-electron transistor device 100 may have numerous designs. Insome embodiments, it is advantageous for a number of the component partsof the single-electron transistor device to be in substantially the sameplane, as illustrated in FIG. 1B. In certain preferred embodiments, forexample, the source and drain 105, 110, quantum island 130 andfixed-gate electrode 135 are all located in substantially the samehorizontal plane. Such configurations are desirable because fabricationis more easily accomplished using conventional processes, such aslithography, as further discussed below. In other embodiments, however,all or a portion of the fixed-gate electrode 135 can be locatedsubstantially out of the plane as the source and drain 105, 110 and thequantum island 120.

As further shown in FIG. 1B, the single-electron transistor device 100may further include a filter 190 configured to allow a drain currenthaving a predefined Coulomb oscillation frequency to pass through thefilter 190. The filter 190 is preferably a high pass, low pass or bandfilter, or combination thereof. Additionally, the filter 190 can beconfigured to allow passage of the drain current having one Coulomboscillation frequency, but not another Coulomb oscillation frequency. Insuch embodiment, for instance, a first logic state is defined when thedrain current passes through the filter 190 while a second logic stateis present when no drain current passes through the filter 190.

Another aspect of the present invention, a method for manufacturing asingle-electron device. FIGS. 3 through 7 illustrate cross sectional,and in some cases, top views, of an exemplary method of fabricating asingle-electron device 300 according to the principles of the presentinvention. One skilled in the art would understand that similarprocedures could be used to form a variety of different single-electrondevices within the scope of the present invention, such as the devicespresented in FIGS. 1A–1B and 2.

Turning first to the cross-sectional view shown in FIG. 3, theillustrated method includes forming a conductive layer 305 over asubstrate 310. The fabrication of the components of the single-electrondevice 300 can include any number of conventional techniques, includinglithographic processes. These processes can be used to deposit a resistmaterial and pattern the conductive layer 305. Exposure of portions ofthe resist to radiation (e.g., ultraviolet or visible light, x-ray, ionbeam, electron beam) followed by conventional etching procedures can beconducted to lithographically define a source 405, drain 410, andquantum island 415, as illustrated in the cross-sectional and top viewsshown in FIGS. 4 and 5, respectively. As further shown in FIGS. 4 and 5,lithographic processes can also be used to form a gate superstructure420 of a fix-gate electrode 425. Of course, this or other devicepatterns can be replicated any number of times to produce a desiredintegrated circuit layout.

One skilled in the art would understand that in other embodiments of themethod, placing the quantum island 415 could be accomplished usingalternative conventional procedures. Such procedures include growing aconductive grain or particle using self-assembled growth procedures,such as molecular beam epitaxy or metal-organic chemical vapordeposition.

Other techniques of placing the quantum island 415 can include isolatingparticular regions of a silicon substrate and subjecting those isolatedregions to an oxidizing process in such a way to isolate the quantumisland 415 from the source and drain, 405, 410 or from the fixed-gateelectrode 425. As illustrated in FIGS. 4 and 5, the oxidizing processalso can advantageously form a dielectric material 430. Analogous tocertain preferred embodiments of the dielectric materials 140, 142 shownin FIGS. 1A–B and 2, the dielectric material 430 can comprise silicondioxide or aluminum oxide.

Turning now to FIG. 6, shown is a top view of the partially completeddevice 300, after doping different regions of the gate superstructure420 of the fixed-gate electrode 425. Conventional lithographic maskingand dopant implantation techniques well known to those skilled in artcan be used to form n-type dopant regions 605 and p-type dopant regions610, and thus define a depletion layer 615 there between, and therebyform a fix gate electrode 425 comprising a pn junction electrode.

With continuing reference to FIGS. 3–5, in other embodiments of themethod, lithographic procedures similar to that described above can beused to pattern the conductive layer 305 to form a fixed gate electrode425 comprising a depletion electrode. As an example, the gatesuperstructure 420 shown in FIG. 5 can be isolated and subjected to anoxidizing process, similar to that described above for isolating thequantum island 415 from the source and drain 405, 410. FIG. 7 presents atop view of the device 300 after completion of the oxidation process,showing a dielectric material 705 that separates first and second bodies710, 715 of the fixed-gate electrode 425.

Yet another embodiment of the present invention, a transistor circuit800, is schematically illustrated in FIG. 8. The transistor circuit 800comprises a single-electron device 810 of the present invention,including a source and drain 815, 820, quantum island 825 and fixed-gateelectrode 830. The single-electron device 810 can comprise any of thepreviously discussed embodiments of the single-electron transistordevices and illustrated in FIGS. 1 through 7. The transistor circuit 800further includes a conventional metal-oxide semiconductor field-effecttransistor (MOSFET) 840 coupled to the single-electron device 810. TheMOSFET 840 is configured to amplify a drain current 850 from thesingle-electron device 810.

One skilled in the art would understand that the transistor circuit 800advantageously improves the voltage gain of the drain current 850 fromthe single-electron device 810 and thereby facilitate the use suchcircuits 800 in forming multiple logic levels. In certain preferredembodiments of the transistor 800, the fixed-gate electrode 830, isconfigured to change a capacitance between the quantum island 825 andthe fixed-gate electrode 830 when a voltage 860 is applied to thefixed-gate electrode 830. In some advantageous embodiments of thetransistor circuit 800, the voltage 860 applied to the fixed-gateelectrode 830 is configured to contain binary information. In stillother preferred embodiments, for example, when the voltage 860 has afirst amplitude, the drain current 850 will have a first Coulomboscillation frequency between about 0.1 and about 1.0 GHz, which, inturn, corresponds to a first logic state. When the voltage 860 has asecond amplitude, the drain current 850 has a second Coulomb oscillationfrequency between about causes said drain current to have a secondCoulomb oscillation frequency of said drain current that is at leastbetween about 2 to 3 times greater than the first Coulomb oscillationfrequency, and which corresponds to a second logic state.

Certain preferred embodiments of the transistor circuit 800, furtherinclude a filter 870 coupled to the single-electron device 810 and theMOSFET 840. As discussed previously, the filter 870 can beadvantageously configured to allow the drain current 850 to pass throughthe filter when the drain current 850 has a predefined Coulomboscillation frequency, and thereby facilitate the defining logic statesin the circuit 800.

Although the present invention has been described in detail, one ofordinary skill in the art should understand that they can make variouschanges, substitutions and alterations herein without departing from thescope of the invention.

1. A single-electron transistor device, comprising: a source and drainsupported by a substrate; a quantum island located between said sourceand drain and that forms tunnel junctions between said source and saiddrain; and a fixed-gate electrode located adjacent said quantum island,said fixed-gate electrode having a capacitance associated therewith thatvaries as a function of an applied voltage, wherein said fixed-gateelectrode comprises a depletion electrode of a metal oxide semiconductorcapacitor.
 2. The single-electron transistor device as recited in claim1, wherein said fixed-gate electrode is configured to change acapacitance between said quantum island and said fixed-gate electrodewhen said voltage is applied to said fixed-gate electrode.
 3. Asingle-electron transistor device, comprising: a source and drainsupported by a substrate; a quantum island located between said sourceand drain and that forms tunnel junctions between said source and saiddrain; and a fixed-gate electrode located adjacent said quantum island,said fixed-gate electrode having a capacitance associated therewith thatvaries as a function of an applied voltage, wherein said fixed-gateelectrode comprises a pn junction electrode.
 4. The single-electrontransistor device as recited in claim 3, wherein said pn junctionelectrode comprises silicon or polysilicon having n-doped regions andp-doped regions.
 5. The single-electron transistor device as recited inclaim 3, wherein said pn junction electrode has a first body adjacentsaid quantum island and a second body united with said first body, along dimension of said second body being perpendicular to a longdimension of said first body.
 6. The single-electron transistor deviceas recited in claim 1, wherein said depletion electrode comprises afirst body adjacent said quantum island and a second body adjacent saidfirst body, said first and second body being separated by a dielectricmaterial.
 7. The single-electron transistor device as recited in claim1, wherein said source and drain, said quantum island and saidfixed-gate electrode are located in a same plane.
 8. The single-electrontransistor device as recited in claim 1, wherein said tunnel junctionsinclude a gap between said source and drain and said quantum island,said gap ranging from about 1 nanometer to about 1000 nanometers, andwherein a dielectric material is located within said gap.
 9. Thesingle-electron transistor device as recited in claim 1, furtherincludes a filter configured to allow a drain current having apredefined Coulomb oscillation frequency to pass through said filter.